Promising Young Woman is a gleaming arsenic lollipop with a razor blade hidden in the middle. It’s enticing and delicious but it also induces queasy wooziness, and then, with the barest warning, it cuts deep. The film is merciless and delightful, defeated and triumphant. It invites the viewer to untangle its politics, only to repel any attempt to condense it to a tidy little thesis. In her feature debut as a writer-director, Emerald Fennel has made a movie that is delirious messy, relentlessly fierce, and grand all around.

In the film, Carey Mulligan plays Cassie, a woman hitting the age of thirty at what appears to be a dead end. Once a rising med student, Cassie works as a barista — with understandable disdain for most of the clientele — and lives at home with increasingly impatient parents who drop passive aggressive hints about moving out and moving on. This is not a standard millennial spinout. Cassie is held in place by enduring animosity over the tragedy that befell a lifelong friend and college classmate. Instinctively closing herself off from most human connection, Cassie regularly patrols flashy nightspots in a well-acted guise of drunken incapacitation, luring in predatory men to ruthlessly, rightly turn the tables on them. It’s not really revenge. It’s more like cosmic justice.

Matters grow more complicated when Cassie encounters Ryan (Bo Burnham), an acquaintance from med school who’s now a pediatric surgeon. After Ryan confesses he always had a crush on her, Cassie’s usual defenses come down and they begin a romance. Initially a somewhat healing experience, the relationship also puts Cassie in proximity of some of the individuals most guilty in the cruel act against her friend. That justice starts to look like something she can mete out to those who are most deserving of it. Soon, she unearths information that makes the need for action seem all the more compelling.

Mulligan meets the film’s fury head on and matches it with her performance. In power and commitment, Mulligan is a titan in the role of Cassie. Even as the role — and Fennell’s comfort with extremes — allows Mulligan to act with bright verve, she’s also shrewd enough to tap into the deep emotions of the character. Unlike most other films that dabble in vengeance, Cassie is not merely a construct, moving forward with easily defined motivation. Mulligan makes her sly, spirited, funny, and rueful. She lives a full life, simply one that’s hobbled by the spiritual wounds she carries.

Fennell gets strong performances out of the entire cast, in part because she daringly casts against type. Up and down the cast list, actors defy expectations, giving Promising Young Woman the happy friction of unpredictability. That sparking energy is then given another shock of contrast by Fennell’s visuals, which are bold and precise. Any suspicion that the movie might be rattling off its rails is countered by the assured craft Fennell brings throughout. The ride gets bumpy because she wants it that way. After all, uncertain and ill turns are encountered by people like Cassie all the time out here on the other side of the screen. Promising Young Woman reflects that and, in its challenge and brio, answers it.